Adjusted AH linear negative binomial regression models exploring the relationship of environmental factors with the number of specimens positive for influenza viruses overall and influenza A and B virusesa

Demographic or climatic factorAdjusted IRR (95% CI) for linear AH model
All influenza virusesInfluenza A virusInfluenza B virus
Age group (yr)
    <11.34 (1.01–1.78)*1.79 (1.23–2.60)*0.71 (0.41–1.22)
    1–42.88 (2.19–3.79)*3.39 (2.35–4.89)*1.80 (1.08–3.00)*
    5–196.76 (4.88–9.36)*7.19 (4.62–11.1)*4.32 (2.37–7.88)*
    20–641.66 (1.38–2.01)*1.92 (1.48–2.48)*1.26 (0.89–1.78)
Outbreak status
    No0.11 (0.09–0.15)*0.07 (0.05–0.11)*0.27 (0.16–0.46)*
AH0.80 (0.78–0.83)*0.79 (0.76–0.82)*0.80 (0.76–0.84)*
WS1.00 (0.98–1.02)0.97 (0.95–1.00)1.03 (0.99–1.07)
Temp fluctuation1.03 (1.01–1.04)*0.99 (0.97–1.02)1.07 (1.04–1.10)*
Temp · temp fluctuation1.00 (1.00–1.01)*1.00 (0.99–1.00)1.00 (1.00–1.00)*
  • a AH, absolute humidity; RH, relative humidity; WS, wind speed; IRR, incidence relative risk, CI, confidence interval. The weekly median was used to measure AH, RH, WS, and temperature. The estimate is considered significant (*) when the 95% confidence interval does not cross 1. The adjusted incidence relative risk of 1.00 indicates the category used as a reference/comparison. The total weekly numbers of positive influenza A and B virus counts were used as dependent variables. The left column indicates all independent/predictable variables for which this model was adjusted. Temperature fluctuation represents the difference in the median temperatures between the exposure day and the previous day. In this model, the weekly median of this measurement was included. Temperature · temperature fluctuation represents the interaction term for which the model was adjusted. These models include measurements of climatic factors and detection of influenza A and B viruses and all influenza virus-positive specimens for 293 unique weeks.